1. Field of the Invention
The present invention relates to a weight calculation method appropriate to suppress spurious elements to detect a reflection signal from a target by means of weight control, a weight calculation device using the weight calculation method, an adaptive array antenna making use of the weight calculation device, and a radar device with the built-in adaptive array antenna.
2. Description of the Related Art
In recent years, an adaptive array antenna has been built into a pulse radar device to carry out so-called adaptive null steering in order to improve target detection precision. The adaptive null steering is a type of processing to form a reception composite beam so that direction-of-arrival (DOA) of unnecessary waves become a null directivity by applying weight control to the phase and amplitude of a reception signal at the adaptive array antenna. For the adaptive array antenna to be used for such a use, it is required to perform the weight control so as to properly form the reception composite beam even under an environment in which a large number of delay signals arrive, or under an environment in which spurious elements such as clutter and interference waves exist.
In the adaptive array antenna, therefore, a weight control method employing a space time adaptive processing (STAP) system has been widely applied. The STAP system has a distinctive feature to improve a signal to interference plus noise ratio (SINR) and enable forming an excellent beam in which directivity in arrival directions of spurious elements are almost zero (null).
The STAP system conducts the following processing. At first, antennas (element antennas, namely channels) in which a plurality of (M pieces of) antennas are arranged in an array form receive targeted reflection signals, and store the reception signals at corresponding cell positions in all processing range cells in which range cells corresponding to reception pulse widths are formed so as to be ranged with prescribed lengths on a time axis. The STAP system carries out an arithmetic operation for a covariance matrix from data of range cells except for range cells (referred to as processing adaptive range cells) in which it is presumed that the target reflection signals are included, namely from data of cells which are supposed to be formed only of spurious elements. Thereafter, the STAP system applies weight control to the antenna reception signals through a beam forming circuit with the use of adaptive weights calculated on the basis of the covariance matrix.
In the weight control in the STAP system, weight calculation for each range cell is performed in a weight calculation circuit so as to calculate the adaptive weights. As to the weight calculation, a multistage wiener filter (MWF) system has been known as a method therefore. In terms of the MWF system, its details are disclosed in the publication entitled IEEE TRANSACTIONS ON INFORMATION THEORY, VOL. 44, No. 7, NOVEMBER 1998 “A Multistage Representation of the Wiener Filter Based on Orthogonal Projections”.
However, despite the fact the reference document related to the foregoing MWF system discloses the weight calculation method to suppress the DOA of unnecessary waves, it does not describe about a decision method of the number of processing stages in the case of application of the weight calculation method. Conventionally, therefore, since a system to decide the processing stages by determining the covariance matrix calculated from the obtained data from a magnitude of an eigenvalue obtained through eigenvalue decomposition is required, this involves excessive calculation.